During drilling of an underwater wellbore, a riser is provided to return the drilling fluid (mud), cuttings and any other solids or fluids from the wellbore to the surface. The drill string extends down the center of the riser, and the returning drilling fluid, cuttings etc. flow along the annular space in the riser around the drill string (the riser annulus).
When drilling of the wellbore is carried out using a floating rig such as a drill ship, a semi-submersible, floating drilling or production platform, it is known to provide the riser with a slip joint which allows the riser to lengthen and shorten as the rig moves up and down as the sea level rises and falls with the tides and the waves. A ball joint (or flex-joint) is also provided to accommodate angular displacement of the riser from the vertical. The returning drilling fluid leaves the riser via a diverter which is mounted above the slip joint.
Such a slip joint is, for example, described in U.S. Pat. No. 4,626,135, and comprises an outer tube section which is connected to the wellhead, and an inner tube section which sits within the outer tube section and which is connected to the rig floor. Seals are provided between the outer and inner tube sections, and these substantially prevent leakage of fluid from the riser whilst allowing the inner tube section to slide relative to the outer tube section.
This system also includes an annular blow out preventer which is located below the slip joint, and which is used as a gas handler to divert the flow of gas from a well control incident. An auxiliary choke line extends from the riser below the point at which the riser annulus is closed by the blowout preventer, and, in the event of such an incident, the blow out preventer is closed, and a valve in an auxiliary choke line opened, so that the formation gas may be circulated out via the auxiliary choke line.
Drilling methods, such as managed pressure drilling (MPD) or mud cap drilling, which involve the pressurization of fluid in the wellbore annulus are becoming increasingly important, and these require the ability to contain fluid pressure in the riser annulus during drilling. Examples of these type of systems are disclosed in U.S. Pat. No. 6,904,981 and U.S. Pat. No. 7,044,237.
In floating drilling rigs, wave motion means that the seals between the outer and inner tube sections of the slip joint are subjected to significant movement, and, as a result the pressure sealing capacity of the seals in conventional slip joint designs is limited. As a result, it is not possible to use the conventional marine riser drilling system described above for MPD.
To address this issue, an alternative system is presented in US2005/0061546 and U.S. Pat. No. 6,913,092, in which a “rotating control head” including rotating blow out preventer (RBOP) or rotating control device (RCD) mounted above the slip joint. Connectors for diverting fluid from the riser are provided on the RCH housing below the RCD/RBOP. When used for MPD, the slip joint is locked to eliminate movement across the slip joint seal, the RCD or RBOP is closed, and fluid returns are directed to the rig's systems (separators, shakers etc.) via hoses connected to the connectors.
A further alternative system is disclosed in WO2011/104279. In this system, the returning drilling fluid exits the riser via a flow spool which is mounted below the RCD and above the slip joint. In this case, a high pressure slip joint is required—an example of a suitable design is disclosed in WO2012/143723.
It is also known to avoid subjecting the flow spool to high pressures during MPD by mounting the RCD/RBOP below the slip joint. Such an alternative system for providing pressurized riser assembly is disclosed in US 2008/0105434. In this system, a universal riser section (OURS) is placed in the riser below the slip joint. The OURS includes, amongst other things, at least one rotating control device (RCD), together with all the usual connections and attachments required to operate the RCD, and at least one outlet for the fluid returns.
It is also known to replace a conventional slip-joint system with a specialized rotating control head system which includes one or more long “flow crosses”—conduits which extend horizontally from the riser, a flow spool or the rotating control device (RCD) housing. Valving and flexible hoses hang from each of the flow crosses, and the mud is returned from the riser annulus via the flow crosses and hoses.
Where fluid lines are connected to riser below the slip joint, the flexibility of the hoses accommodates the heave of the rig floor relative to the riser.